1. Field of the Invention
The present invention relates to radio frequency pulse generators, and more particularly to high power, solid state burst generators in the microwave region.
2. Description of the Prior Art
There are many applications in which it is useful to be able to generate short, high power (megawatt) bursts of radio frequency power. For example, radio frequency bursts are useful in high resolution radar, time domain meteorology, microwave generation, countermeasure devices, military weapons, and plasma research and diagnosis. Many conventional systems for generating radio frequency bursts have experienced serious problems because of the relatively high losses and other inefficiencies inherent in those systems. One type of prior art system in this class involves the direct DC to RF conversion at megawatt power levels. One such device, called the frozen wave generator (FWG), was specifically designed to avoid the many problems unique to the generation of very high peak power bursts by the direct conversion of DC to RF power. For instance, the FWG is a device that employs very fast switches and as such has made use of optoelectronic techniques to perform the switching functions. The recently developed photoconductive switches (PCS) are ideally suited to be used in the FWG. These switches are disclosed by L. Bovino, M. Weiner and T. Burke in "Optically Controlled, High Power, High Speed Semiconductor Switches", ELECTRO '87, Professional Program Record, Session 24, pp. 1-7, incorporated herein by reference. Also, incorporated herein by reference is a publication concerning the FWG by Chi H. Lee et al entitled "Optoelectronic Techniques for Microwave and Millimeter-Wave Applications", 1985 IEEE MTT-S International Microwave Symposium, pp. 178-181.
In order to make the FWG work effectively, certain critical switch characteristics must be achieved. First, the switch closing time (pulse rise time) must be significantly less than one half-cycle in order not to interfere with the FWG's characteristic frequency. In addition, the simultaneity of the switches should be very good; the overall switch jitter must be short, i.e. all switches must close within a short period. These criteria can be met by many low power switches at high frequency and several high power switches at low frequency. The bulk PCS is probably the only device that can meet the rise time and jitter requirements of a megawatt FWG at GHz frequencies. Further, in the FWG, switch conduction losses add making it difficult to construct a reasonably efficient FWG having a switch on-time that is long. Although a bulk PCS can be designed which will meet the above requirements, at the higher frequencies and longer burst durations, the requirement for fast rise time combined with long on-time (at low loss) becomes extremely difficult to meet. A high power PCS requires a relatively high power laser as a source. Also, the PCS is least lossy while the laser remains on, therefore, a fast-rising, long pulse width, high-power laser is the optimum source. Consequently, the design of the FWG is severely limited by the availability of a suitable laser to close the PCS. In addition, the preferred technique for closing the switches simultaneously is to use a single laser pulse to drive all the switches. Using this technique holds some switches closed for longer than is necessary, thus wasting laser energy. Again, the problem becomes more severe at longer burst lengths. As the laser is usually the least efficient component of the system, it would be advantageous to devise a periodic waveform generator which is not as wasteful as the FWG.
Therefore, although the FWG and other burst generators have, in general, served the purpose, they have not proved entirely satisfactory, having a number of deficiencies such as those described above. As such, those concerned with the development of RF burst generators have recognized the need for improvements therein. The present invention fulfills this need.